1. Field of the Invention
The present invention relates to a zoom lens system for a microscope and a microscope using that zoom lens system. More particularly, the present invention relates to a zoom lens system that is to be applied to an infinity correction type microscope and forms an image directly on an image pickup element, such as a CCD, without forming an intermediate image, and a microscope using that zoom lens system.
2. Related Background Art
Conventionally, there has been used microscope zoom lens systems using a relay lens system in which an intermediate image is formed by an imaging lens utilizing a light flux from an objective lens and the magnification of the intermediate image is varied by a relay lens having a zoom function (see, for example, Japanese Patent Application Laid-Open No. 9-274137).
However, in the aforementioned microscope zoom lens system using a relay lens system, an intermediate image is once formed by an imaging lens and then the intermediate image is relayed by a zoom relay lens onto an image pickup element. This results in the problem that the optical system becomes long and when the system is built in a microscope, the size of the microscope apparatus becomes large. In addition, since it is necessary to provide a positive field lens for relaying a pupil position to a position in the vicinity of the intermediate image, the Petzval sum of the optical system as a whole becomes positive and large and a negative curvature of field is generated, which is undesirable.